In a typical mask making process, all defects that are found on the photomask during the inspection process must be repaired. Most criteria classify defects based on size. Accordingly, inspection tools are tuned to only finding defects that are larger than a specified minimum defect size. It is known in the art that not all defects cause problems during the lithography process. A defect on a photomask is deemed harmful only if the image of the defect is transferred to the wafer during the lithographic printing process. If the defect is not imaged onto the wafer, then the mask maker may not be required to repair the defect altogether, thereby saving time and money. Moreover, if the defect is imaged onto the wafer in a way that it does not adversely affect the performance of the wafer, then this defect is preferably left untouched.
The difficulty in classifying defects as described is that the mask maker is not typically provided with a good methodology for determining what defects will adversely affect the wafer. Defects that are close to a critical feature may have an impact on the feature depending on several factors that include: feature type, proximity of the defect to the feature and the size, shape, phase and transmission of the defect. These effects are further complicated when enhancement techniques, such as phase-shifting masks and optical proximity correction, are utilized. Since defects are traditionally classified by type (e.g., a hole in the chrome) as well as by their size (relative to the critical feature size on the mask), current defect specifications are becoming increasingly difficult to be met, as the size of the critical features on the mask continues to shrink to below 1.0 .mu.m. This difficulty will likely drive the need for new and more expensive inspection and repair tools, and will lower the achievable yields on advanced photomasks. The end result of the upgrade in tooling translates into an increased cost per mask.
Since photomask inspection tools typically use an optical system that is significantly different than the lithography exposure system, the inspection tool cannot in general predict the effect of the defect on the image transferred to the wafer. It is believed, however, that the inspection systems can find most defects that negatively impact the wafer performance along with those that do not. Since repairs are expensive, certain categories of defects cannot be repaired and some repairs may end up making the defect worse, it is desirable to concentrate on those defects that ultimately will cause problems on the wafer.
To understand the methodology of the invention, one must first have a basic understanding of the "process window" for lithographically printing a feature. The process window for a given feature is the amount of variation in the process that can be tolerated while still maintaining critical aspects of that feature within accepted tolerances from their desired values. In lithography, the process window is normally stated by the amount of focus and exposure dose variation that can be tolerated while maintaining feature sizes and critical dimensions (CD) within a given tolerance of their nominal values.
Process windows are typically found by either taking CD measurements on wafers that have been exposed at various focus and exposure conditions, a focus-exposure matrix (FEM), or by computing the CD from through-focus intensity profiles. In the latter case, the exposure dose can effectively be varied by changing the intensity value at which the CD is measured. These intensity profiles are typically generated either by simulation or by recording through an aerial image measurement system (AIMS) that emulates the lithography exposure conditions. The AIMS typically consists of a microscope that has a numerical aperture and illumination conditions that emulate the lithography exposure conditions. This system records the aerial image, or the image of the photomask that is projected onto the photoresist by the lithography exposure tool.
FIG. 1a shows the intensity profile through a bright-field isolated line taken at five focus conditions. From this figure, the dimension of the line that prints in the photoresist can be found by drawing a line across the intensity plots at a constant intensity level, (e.g., line 101). Assuming that the photoresist has a threshold response to the light intensity impinging upon it, the printed dimension can be determined by finding the intersections of the intensity curve for a given focus and the constant intensity line, such as points 102 and 103 for the 0.0 .mu.m defocus line, and points 104 and 105 for the 0.5 .mu.m defocus line.
In all the regions where the intensity level exceeds the threshold level, for a positive tone photoresist, the photoresist undergoes a photochemical reaction that allows it to be removed with a developer solution. All the regions having intensities below the threshold level are resistant to the developer solution and remain intact on the wafer after the development process. In FIG. 1a, the intensity of the light reaching the photoresist has been normalized by the intensity of light in a large clear area, I0. In a threshold resist model, if the dose-to-clear (DTC) is defined as the dose required to just develop away a large clear area, then the dose needed to print at a given threshold level is determined by the inverse of the normalized intensity at that threshold multiplied by the DTC. For example, if the exposure dose is given by 4.0*DTC, the normalized intensity at the threshold level is 1/4.0=0.25. It is understood that not all photoresists have a threshold response to the incident light intensity. Nonetheless, most photoresists can be analyzed using a similar method.
Practitioners in the art will fully realize that for an exposure dose set at 4.0*DTC , the threshold for exposure will be 0.25, which approximates the threshold drawn by line 101. For this exposure dose, the size of the line printed in photoresist will be determined by the separation between points 103 and 102, or about 0.25 .mu.m, for the 0.0 .mu.m defocus condition, and by the separation between points 105 and 104, or about 0.17 .mu.m for the 0.5 .mu.m defocus condition. Likewise, for a lower exposure dose set at 2.0*DTC, the exposure threshold will be given by an intensity value of 0.5, which in FIG. 1a corresponds to line 106. Therein, the printed dimension remains basically constant for defocus positions between 0.0 and 0.5 .mu.m. This dimension, approximately 0.35 .mu.m, is defined by the separation between points 108 and 107.
From the CD measurements, one can find functional dependencies of feature size versus focus and exposure dose. FIG. 1b depicts a plot of the line width versus focus at various doses for the isolated line. On this plot, point 111 corresponds to the line width for an exposure dose of 4.0*DTC and a defocus of 0.0 .mu.m, which is equivalent to the separation between points 103 and 102 (FIG. 1a). Likewise, point 112 is the line width for the same dose and defocus of 0.5 .mu.m, which is the separation between points 105 and 104 (FIG. 1a). Points 113 and 114 are the line widths found for a dose of 2.0*DTC at defocus values of 0.0 and 0.5 .mu.m, respectively.
The line width versus focus and dose functions can now be used to determine the amount of focus and dose variation that can be tolerated by the lithography system while maintaining the feature size within the desired tolerance from its nominal size. This is one representation of the process window of the feature. An illustration applicable to an isolated line is depicted in FIG. 1c. Shown therein is the focus value plotted on the vertical axis against the logarithm of the dose on the horizontal axis. Curve 121 represents focus and dose conditions wherein the dimension of the line printed in the photoresist equals the target dimension, 0.25 .mu.m, to which the tolerance is added, e.g., 10% of 0.25 .mu.m or 0.025 .mu.m. In this case, curve 121 represents the conditions where the line prints at 0.25+0.025=0.275 .mu.m. Curve 122 represents the conditions where the line prints at the target minus the tolerance, i.e., 0.25-0.025=0.225 .mu.m. The region between the curves represents the focus and dose conditions where the line is printed in photoresist within the desired tolerance of its nominal size. Likewise, the common process window of two features can be determined by how much focus and dose variation can be tolerated while still maintaining both features within an acceptable tolerance from their nominal sizes.
The process window for a given tolerance can also be stated as the amount of focus variation that can be tolerated, commonly referred to as the depth-of-focus (DOF), and which is a function of the amount of exposure dose variation, or exposure latitude (EL). FIG. 1d shows a representation of the process window for the isolated line. This curve is often found by determining the largest rectangles that are contained within the positive and negative tolerance lines shown in FIG. 1c. In this case, the horizontal dimension of the rectangle corresponds to a given dose variation. The vertical dimension represents the amount of focus variation that can be tolerated when the given amount of dose variation is present while maintaining the line width within tolerance. In FIG. 1c, rectangle 123 represents a 25% dose variation, and its vertical dimension corresponds to a focus variation of 0.6 .mu.m. In FIG. 1d, point 124 corresponds to a 0.6 .mu.m DOF at 25% EL. Since it is more convenient to specify a process window in terms of a single number, the process window will often be specified as either the depth of focus at 10% exposure latitude or the amount of area under the DOF versus the EL curve, which is commonly known as the total window. Other values can also be used as a measure of the process window.
FIG. 2 outlines a conventional process used by a device manufacturer to set a mask defect specification followed by the mask manufacturer using that specification during the inspection/repair process.
In the conventional process that defines a defect specification, the device manufacturer determines the wafer nominal CD and CD tolerance in the manner to be described hereinafter with reference to FIG. 4. However, the mask CD tolerance is normally set as only some fraction of the wafer CD tolerance (204). By way of example, the mask CD tolerance is often set at one-third times the exposure tool magnification of the wafer CD tolerance, or 4/3 times the wafer CD tolerance for a 4.times. exposure tool. Accordingly, if the wafer CD tolerance is 30 nm, the mask CD tolerance will be set at 40 nm for a 4.times. mask. This fractional method is based on apportioning a certain fraction of the total error budget to different parts of the process. Herein, the mask error is allocated one-third of the total error budget, variations in the lithography process are allocated another one-third, and all remaining process variations are apportioned the final third. This method is generally valid for printing features having a k1 factor larger than about 0.5, wherein:
k1= feature size * stepper numerical aperture/wavelength of the exposure illumination.
The method of allocating an error budget becomes invalid for imaging where k1 drops below 0.5 due to the amplification of mask errors by the lithographic process.
Referring again to FIG. 2, mask defects are, typically, only specified by their size, and this size is taken as a fraction of the nominal mask critical dimension (206). For instance, it is normal to consider as defects any undesired structures on the mask having a dimension greater than 10% of the nominal mask dimension. For a mask with nominally 1.2 .mu.m features, all undesired features that are larger than 120 nm are considered defects. Since the wafer CD tolerance specification for these types of features is usually about 10% of the nominal wafer dimension, the wafer CD tolerance for these features will be approximately (1.20 .mu.m/4) * 0.1=30nm. Then, the mask CD specification is typically set at 4/3.times.30 =40nm. This indicates that the defect criterion does not correlate well to the mask CD specification. By allowing the defect specification to be larger than the CD specification, the device manufacturer has traditionally supported the concept that defects do not impact the wafer image as much as a CD variation. The conventional method of quantifying this concept is, however, somewhat arbitrary.
The standard mask inspection/repair process entails incorporating the defect size criterion of the device manufacturer into the inspection tool (208). The tool then locates all undesired features that are larger than that predetermined defect size. Each defect is then reviewed and classified. If a defect is considered non-repairable, then a judgment call (211) is made to establish whether or not the defect will print on the wafer. Advanced mask makers may utilize the AIMS tool to facilitate this process, but it is standard practice with an AIMS review to base this decision strictly on a comparison of the peak intensity values for defective and non-defective features. The behavior of the defect at various dose and focus conditions is not normally considered. If the defect is not repairable and is believed to negatively impact a critical feature on the wafer, the mask is routinely scrapped (212). After review, all repairable defects are typically repaired (210) and the mask finished and shipped (214).